Our overall aim is to study the biosynthesis of transmitter substances and related subcellular constituents of identified neurons in the marine mollusc, Aplysia californica. The metabolism of a transmitter substance seems to be a characteristic property of a differentiated nerve cell. The molecular mechanisms involved in transmitter metabolism are central to understanding neuronal function, and might best be approached at the level of the single neuron. We intend to use our recently developed technique of intracellular injection in order to introduce radioactively labeled substances of high specific activity and other pharmacologically active substances directly into cell bodies of identified neurons. Thus we can study: 1) the biosynthesis of transmitter substances in identified neurons under physiological conditions; 2) axonal transport; and 3) the morphology of identified synapses.